Single mask screening process and structure produced thereby

ABSTRACT

A process of forming a multi-layer feature on a ceramic or organic article in which first and second layers of paste are sequentially screened through a screening mask wherein the screening mask has not been moved between screening steps. A structure produced by this process is also disclosed.

RELATED APPLICATION

[0001] This application is related to Buechele et al. U.S. patentapplication Ser. No. ______ entitled “A SCREENING APPARATUS INCLUDING ADUAL RESERVOIR DISPENSING ASSEMBLY” filed even date herewith, thedisclosure of which is incorporated by reference herein.

BACKGROUND OF THE INVENTION

[0002] This invention relates to electronic substrates, and moreparticularly, relates to ceramic and organic substrates havingmultilayer features thereon for electronics packaging applications and amethod for forming such multilayer features.

[0003] Glass, ceramic and glass ceramic (hereafter just ceramic) andorganic structures are used in the production of electronic substratesand devices for electronics packaging applications. Many different typesof structures can be used. For example, a multilayered ceramic circuitsubstrate may comprise patterned metal layers which act as electricalconductors sandwiched between ceramic layers which act as insulators.Organic substrates, also coalled printed circuit boards, may be singlelayer or multilayer material (such as fiberglass-impregnated epoxy) andcontain electrical conductors. The substrates are designed withtermination pads for attaching semiconductor chips, capacitors,resistors, connection leads, pins, solder balls, solder columns etc.Interconnection between buried conductor levels in ceramic substratescan be achieved through vias formed by metal paste-filled holes in theindividual ceramic layers (called greensheets) formed prior tolamination, which, upon sintering will become a sintered dense metalinterconnection of metal based conductor. In the case of organicsubstrates, interconnection between conductor levels is by, for example,plated through hole vias.

[0004] The termination pads are often multi-layered stacks ofmetallization and are conventionally produced with multiple screenings,with the underlying layer being screened and dried before application ofanother mask and screening and drying of the next layer. Greenstein U.S.Pat. No. 4,025,669, Siuta U.S. Pat. 5,202,153, and Knickerbocker et al.U.S. Pat. No. 5,293,504, the disclosures of which are incorporated byreference herein, are examples where multiple screenings have beenutilized to obtain either a thicker layer or a multiple layer stack ofmetallization.

[0005] While the prior art shows the individual layers of the stack tobe perfectly aligned with every other layer in the stack, the reality isvery far from this ideal case. For example, Natarajan et al. U.S. Pat.No. 5,639,562, the disclosure of which is incorporated by referenceherein, shows a two layer composite metal pad with both layers perfectlyaligned.

[0006] Gaynes et al. U.S. Pat. No. 5,565,033, the disclosure of which isincorporated by reference herein, discloses a process for making thickerlayers of solder pastes and conductive adhesives. Gaynes et al.recognizes the disadvantages of multiple screenings as contaminationbetween successively screened layers and the time associated with twopasses through screening and drying.

[0007] In practice, the individual layers may be shifted from the layerabove or below it. Mitani et al. U.S. Pat. No. 4,324,815, the disclosureof which is incorporated by reference herein, recognizes the positionalerror that can occur with each printing step. As disclosed in Mitani etal., the bottom layer could be made larger than the top layer so thatthe top layer is “captured” by the bottom layer.

[0008] However, with the trend to increasing the density of thetermination pads (and reducing the spacing between adjoining terminationpads), it is no longer possible to oversize the bottom layer to capturethe top layer. For example, a typical pin grid array substrate has anominal pad diameter of 1.5 mm, an interpad space of 0.3 mm and a padtolerance of +0/−0.220 mm. This is to be compared with a typical columngrid array substrate which has a nominal pad diameter of 0.800 mm, aninterpad space of 0.200 mm and a pad tolerance of +/−0.050 mm.

[0009] Accordingly, it is a purpose of the present invention to have animproved process for producing multilayer stacks of metallization on aceramic article and/or organic article.

[0010] It is another purpose of the present invention to have animproved process for producing multilayer stacks of metallization on aceramic article and/or organic article which eliminates the positionalerrors which heretofore have been inherent in multiple screenings ofmetallization.

[0011] It is yet another purpose of the present invention to have animproved process for producing multilayer stacks of metallization foruse as termination pads, lines and other features.

BRIEF SUMMARY OF THE INVENTION

[0012] One aspect of the invention relates to a method of forming amulti-layer feature on an electronic substrate article, the methodcomprising the steps of:

[0013] a. placing a screening mask having at least one aperture over theelectronic substrate article;

[0014] b. screening a first paste through the at least one aperture ofthe screening mask to form a first layer of the multi-layer feature;

[0015] c. screening a second paste through the same at least oneaperture of the same screening mask used in the first screening step andonto the first paste to form a second layer of the multi-layer featurein alignment with the first layer of the multi-layer feature, whereinthe screening mask has not been moved between the two screening steps;and

[0016] d. removing the screening mask.

[0017] A second aspect of the invention relates to a multi-layer featurestructure on an electronic substrate article comprising:

[0018] an electronic substrate article; and

[0019] a multi-layer feature structure comprising:

[0020] a first portion adjacent to the electronic substrate article; and

[0021] a second portion having a bottom and a periphery wherein thefirst portion contacts the bottom, and surrounds the periphery of, thesecond portion so that the second portion is captured by the firstportion.

BRIEF DESCRIPTION OF THE DRAWINGS

[0022] The features of the invention believed to be novel and theelements characteristic of the invention are set forth withparticularity in the appended claims. The Figures are for illustrationpurposes only and are not drawn to scale. The invention itself, however,both as to organization and method of operation, may best be understoodby reference to the detailed description which follows taken inconjunction with the accompanying drawings in which:

[0023]FIG. 1 is a side view of a prior art multilayer pad in which thetop layer is misaligned with respect to the bottom layer.

[0024]FIG. 2 is a side view of a prior art multilayer pad in which thebottom layer has been made larger to capture the top layer.

[0025]FIG. 3 is a flow chart showing the process flow for the prior artprocess of multiple screenings to form a multilayer pad.

[0026]FIG. 4 is a flow chart showing the process flow for the processaccording to the present invention of multiple screenings to form amultilayer feature.

[0027]FIGS. 5A, 5B and 5C are schematical cross-sectional views showingthe different steps in producing the multilayer feature according to thepresent invention.

[0028]FIGS. 6A and 6B are top views of the multilayer feature producedaccording to the present invention.

[0029]FIG. 7 is a schematical cross-sectional view of a secondmultilayer feature produced according to the present invention.

[0030]FIG. 8 is a schematical cross-sectional view of a third multilayerfeature produced according to the present invention.

DETAILED DESCRIPTION OF THE INVENTION

[0031] Referring to the drawings in more detail, and particularlyreferring to FIG. 1, multilayer pad 12 has been formed on ceramicsubstrate 10. Due to positional error resulting from multiplescreenings, top layer 16 is not aligned with bottom layer 14. Themisalignment results in portion 18 of top layer 16 contacting ceramicsubstrate 10. The consequences of this structure are that a terminationpad of reduced dimension has been formed and that if top layer does notbond well with ceramic substrate 10, the misalignment can causedelamination or spalling of top layer 16.

[0032] In FIG. 2, multilayer pad 12 on ceramic substrate 10 has beenformed with bottom layer 14 larger than top layer 16. In this way, fullcontact between top layer 16 and bottom layer 14 is assured even ifthere is positional error from the multiple screenings. A disadvantageof this arrangement is that excess space has been taken up by themultilayer pad 12, thereby reducing the possible density of theremaining multilayer pads (not shown) on the ceramic substrate.

[0033] The prior art process for producing a two-layer multilayer pad isillustrated in FIG. 3. The first layer is screened 20 through a maskonto a ceramic greensheet. The mask is removed 22 followed by drying 24and inspection 26 of the screened features. Thereafter, the second layeris screened 28 through a mask onto the first layer. The mask is thenremoved 30 followed by drying 32 and inspection 34 of the screenedfeatures. Finally the greensheet is stacked with other ceramicgreensheets, laminated and sintered.

[0034] It doesn't matter whether the same mask or different masks areused in the prior art process to screen both of the layers, the resultwill be the same, namely, misalignment between the two layers due topositional error.

[0035] Referring now to FIG. 4, the process according to the presentinvention is illustrated. The first layer of the multilayer feature isscreened 36 onto an electronic substrate article with the mask. Withoutmoving the mask, the second layer is screened 38 through the same mask.

[0036] Referring now to FIG. 5A, mask 48 having feature opening 50 inline with via 47 is situated on electronic substrate article 46. Mask 48is preferably a metal mask suitable for extrusion screening but it iscontemplated within the scope of the present invention that a mesh masksuitable for silk screen printing could also be used, although it isclearly not preferred because of the inherent movement of the mesh maskduring the silk screen printing process. Nozzle 52 is poised to providea quantity of paste for screening.

[0037] The preferred screening apparatus, including the nozzle, isdescribed in more detail in Applicants' RELATED APPLICATION. Nozzle 52should be relatively pliable for reasons which will become apparenthereafter. Suitable materials for nozzle 52 could be polyurethane,elastomers, thermoplastics, natural rubber, silicone, TEFLON (TEFLON, atetrafluoroethylene (TFE) compound is a trademark of E.I. duPont deNemours & Co.) or other similar materials.

[0038] Nozzle 52 will screen the paste into feature opening 50. Due tothe pliable nature of the nozzle 52, part of the screened paste isscooped out by the pliable nozzle 52 leaving layer 54 only partiallyfilling the feature opening 50. Nozzle 52 will also wipe the mask,removing any paste residue. The resulting unfilled portion 56 of featureopening 50 will be filled by the second layer. If unfilled portion 56were not present, there would be no place for the second layer to fillin the feature opening 50. As can be seen in FIG. 5B, the first layer 54will fill the bottom portion of feature opening 50 and will also usuallyfill the top portion of feature opening 50 along the walls of featureopening 50. The resulting unfilled portion 56 is dish-shaped.

[0039] It is not necessary to dry layer 54 before proceeding to the nextscreening step.

[0040] Next, nozzle 58 will screen the second layer 60 of paste intofeature opening 50 to fill it as illustrated in FIG. 5C. Nozzle 58 ischosen to be made of a hard material such as a carbide, tool steel orceramic material. The purpose of nozzle 58 is to fill the remainingportion 56 of feature opening 50. If nozzle 58 were pliable like nozzle52, paste would be screened into feature opening 50 and then scooped outso that the feature opening would remain unfilled.

[0041] The pastes used are preferably metal-containing (conducting)pastes although insulative (nonconducting) paste or pastes, orcombinations of metal-containing pastes and insulative pastes, could beused for a given application.

[0042] Also, the pastes for the different layers will typically differfrom layer to layer so that the most advantageous properties can bedeveloped for each layer. That is, the first layer 54 may be formulatedfor adhesion to the electronic substrate article 46 while the secondlayer 60 may be formulated for soldering or brazing.

[0043] The present invention is most suitable for a two-layer multilayerfeature. It is possible, however, by varying the hardness of the nozzles52, 58 to build up three or more layers according to the presentinvention.

[0044] Referring back to FIG. 4, the mask is removed 40 followed bydrying 42 and inspection 44 of the screened features.

[0045] In a preferred embodiment of the present invention, themultilayer feature shown in FIGS. 5A, 5B and 5C is a multilayertermination pad formed on a ceramic greensheet 46. Subsequent to thestep of inspecting, the ceramic greensheet 46 with the multilayertermination pad would be stacked with other appropriate greensheetshaving appropriate wiring features (as is well known to those skilled inthe art), laminated under pressure and then sintered to form amultilayer ceramic substrate having at least one multilayer terminationpad.

[0046] The resulting multilayer feature is shown in FIG. 6A prior tolaminating and in FIG. 6B after laminating and sintering. As a result oflaminating, the exposed part of first layer 54 becomes greater. As canbe seen, first layer 54 completely captures second layer 60. Due to thefact that the screening mask is never moved between screening steps,there is complete alignment between first layer 54 and second layer 60.This would be true even if first layer 54 did not rise up along thewalls of the feature opening as shown in FIG. 5B.

[0047] Surprisingly, there is little or no contamination of the firstlayer 54 by the second layer 60 even though the paste for the secondlayer 60 can be screened on while the paste for the first layer 54 isstill wet. The fact that first nozzle 52 wipes the mask 48 afterscreening is useful in avoiding intermixing of the first layer 54 andthe second layer 60.

[0048] Instead of forming the multilayer termination pads on ceramicgreensheets, the multilayer termination pads could be formed on agreensheet laminate, i.e. a stack of laminated greensheets, a sinteredceramic substrate, or an organic substrate, in which case the processflow of FIG. 4 would still apply. After inspection, the greensheetlaminate and multilayer termination pads would be sintered while thesintered ceramic substrate and multilayer termination pads would beresintered. As to the organic substrate the preferred multilayertermination pad would comprise solder paste. After inspection, theorganic substrate and multilayer termination pad would be heated tocause reflow of the solder.

[0049] Too, instead of forming multilayer termination pads, the presentinvention could be utilized to form other multilayer features.Illustrated in FIG. 7 is another embodiment of the present invention inwhich multilayer lines or wiring patterns 62, one of which contacts via47, are formed-on ceramic article 46. Illustrated in FIG. 8 are ceramicarticles 46, preferably ceramic greensheets, wherein one ceramic article46 contains a multilayer feature 64 which in conjunction with via 47forms an in-line capacitor. In practice, the two ceramic articles 46 inFIG. 8 would be adhered together, or in the case of ceramic greensheets,would be laminated and sintered.

[0050] It will be apparent to those skilled in the art having regard tothis disclosure that other modifications of this invention beyond thoseembodiments specifically described here may be made without departingfrom the spirit of the invention. Accordingly, such modifications areconsidered within the scope of the invention as limited solely by theappended claims.

What is claimed is:
 1. A method of forming a multi-layer feature on anelectronic substrate article, the method comprising the steps of: a.placing a screening mask having at least one aperture over theelectronic substrate article; b. screening a first paste through the atleast one aperture of the screening mask to form a first layer of themulti-layer feature; c. screening a second paste through the same atleast one aperture of the same screening mask used in the firstscreening step and onto the first paste to form a second layer of themulti-layer feature in alignment with the first layer of the multi-layerfeature, wherein the screening mask has not been moved between the twoscreening steps; and d. removing the screening mask.
 2. The method ofclaim 1 further comprising the step of heating the multi-layer feature.3. The method of claim 1 wherein in the first screening step, the pasteonly partially fills the at least one aperture of the screening mask. 4.The method of claim 3 wherein in the second screening step, the pastefills the remainder of the at least one aperture of the screening mask.5. The method of claim 1 wherein the first paste is wet when the secondpaste is screened onto it.
 6. The method of claim 1 wherein the firstand second pastes are metal-containing pastes.
 7. The method of claim 1wherein the first and second pastes are insulative pastes.
 8. The methodof claim 1 wherein one of the first and second pastes is ametal-containing paste and the other of the first and second pastes isan insulative paste.
 9. The method of claim 1 wherein the electronicsubstrate article is an organic article.
 10. The method of claim 2wherein the electronic substrate article is an organic article, thefirst and second pastes comprise solder and the step of heating themulti-layer feature causes reflow of the solder layers.
 11. The methodof claim 2 wherein the electronic substrate article is a ceramicsubstrate and the step of heating causes sintering of the multi-layerfeature.
 12. The method of claim 11 wherein the ceramic substrate is aceramic greensheet and the method further comprises, prior to the stepof heating: screening a paste onto at least a second ceramic greensheetto form a wiring feature; removing the screening mask; and stacking andlaminating the ceramic greensheets.
 13. The method of claim 11 whereinthe ceramic substrate is a ceramic greensheet laminate.
 14. The methodof claim 11 wherein the ceramic substrate is a sintered ceramicsubstrate.
 15. The method of claim 1 wherein the screening mask is ametal mask.
 16. The method of claim I wherein the screening mask is thatused in silk screen printing.
 17. The method of claim 1 wherein thefirst and second steps of screening are by extrusion screening.
 18. Themethod of claim 1 wherein the first and second steps of screening are bysilk screen printing.
 19. The method of claim 1 wherein the multi-layerfeature is a termination pad.
 20. The method of claim 1 wherein themulti-layer feature is a wiring line.
 21. The method of claim 1 whereinthe multi-layer feature is part of a capacitor.
 22. A multi-layerfeature structure on an electronic substrate article comprising: anelectronic substrate article; and a multi-layer feature structurecomprising: a first portion adjacent to the electronic substratearticle; and a second portion having a bottom and a periphery whereinthe first portion contacts the bottom, and surrounds the periphery of,the second portion so that the second portion is captured by the firstportion.
 23. The multi-layer feature structure of claim 22 wherein themulti-layer feature is a termination pad.
 24. The multi-layer featurestructure of claim 22 wherein the multi-layer feature is a wiring line.25. The multi-layer feature structure of claim 22 wherein themulti-layer feature is part of a capacitor.
 26. The multi-layer featurestructure of claim 22 wherein the electronic substrate article is anorganic substrate.
 27. The multi-layer feature structure of claim 22wherein the electronic substrate article is a ceramic greensheet. 28.The multi-layer feature structure of claim 22 wherein the electronicsubstrate article is a ceramic greensheet laminate.
 29. The multi-layerfeature structure of claim 22 wherein the electronic substrate articleis a sintered ceramic substrate.